Revise GCSE Physics

Yes, without diffraction mountains would cast radiowave shadows into valleys and you could only receive a radio signal if you were on line of sight with a radio mast.

Fortunately radio waves have very long wavelengths and can diffract down into valleys and around large buildings. So diffraction makes radio communications much easier because you do not need to be on 'line of sight' with the emitter in order to receive the signal.

Diffraction is also the reason that we can hear sounds around corners. Sound waves have a much longer wavelength than light waves, we can hear someone coming around a corner before we can see them, because sound diffracts well but light hardly diffracts at all!